Proterozoic

A long time ago, Earth changed a lot.

More air grew in the sky. This happened because of how the ground and water changed. This new air helped new life start to grow.

Some parts of the world even froze. The Earth became a big ball of ice.

Big lands also moved around. They crashed together to make mountains. They broke apart to make new seas.

Small life forms began to live together. We can find their old shapes in rocks.

It was a very busy time for our world.

The Proterozoic was a very long time in Earth's history. It was the longest era of time. During this time, the air changed. Oxygen began to build up in the sky. This happened because minerals in the ocean were used up. Before this, oxygen stayed trapped in the water. This change is called the Great Oxygenation Event. It was a big event that changed life.

Earth's land was also busy. Big pieces of land moved around. They crashed together to make mountains. This is called orogeny. These lands formed a supercontinent called Rodinia.

At times, the world grew very cold. Some scientists think the Earth became a ball of ice. This is called Snowball Earth.

Life also changed in new ways. Small, single cells became more complex. We call these eukaryotes. These life forms grew into larger groups. They had soft bodies. We find their fossils in rocks from the Ediacaran period. These fossils show us the first clear signs of life.

The Proterozoic was a very long part of Earth's history. It is the third of four geologic eons. It is also the longest eon in the whole geologic time scale. The name comes from Greek words meaning "earlier life." This time began when free oxygen appeared in the air. It ended just before complex life grew very fast. This era is part of the older Precambrian group.

One major thing that happened was the rise of oxygen. For a long time, oxygen could not build up in the air. It was trapped by minerals like iron and sulfur in the ocean. These minerals acted like a sponge for the oxygen. Once these minerals were used up, oxygen began to fill the atmosphere. This first big surge is called the Great Oxygenation Event. It also caused a mass extinction of life that did not like oxygen.

Earth's surface was also very active during this time. Huge pieces of the Earth's crust moved and crashed together. This movement is called orogeny, which is a way to say mountain building. These movements helped form the first supercontinents. One famous supercontinent was named Rodinia. It formed around 1000 to 750 million years ago. Rodinia was made of many pieces attached to a core called Laurentia.

Scientists use rocks to learn about this long period. They look at many layers of rock in shallow seas. These rocks are often easier to study than older ones. We can see evidence of many ice ages in these layers. Some scientists think the Earth became a giant ball of ice. They call this the Snowball Earth idea. This might have happened during the Sturtian or Marinoan glaciations.

Life changed a lot during the Proterozoic. Simple cells evolved into more complex cells called eukaryotes. These cells grew into bigger living things with soft bodies. We can find their fossils in the Ediacaran period. These fossils include things like sponges and algae. Some of these creatures even had sexual reproduction. This period led directly to the many animals we see today.

The Proterozoic is the third of Earth's four geologic eons. It is the longest eon in the entire geologic time scale. The name comes from Greek words meaning "earlier life." This eon follows the Archean and precedes the Phanerozoic. It is part of the larger Precambrian supereon. This era covers the time from the first appearance of free oxygen to the rise of complex life.

One of the most important changes was the rise of atmospheric oxygen. For a long time, oxygen could not build up in the air. It was trapped by mineral sinks of unoxidized sulfur and iron. These minerals acted like a sponge for the oxygen. Banded iron formations show where this process happened. These formations provide most of the world's iron ore today. The accumulation of these iron oxides ceased after 1.9 Ga.

The first massive surge in oxygen is called the Great Oxygenation Event. Some scientists also call it the Oxygen Catastrophe. This name reflects a mass extinction of almost all life at the time. These organisms were obligate anaerobes, meaning they could not survive with oxygen. A second rise occurred later during the Neoproterozoic Oxygenation Event. This second surge helped drive the rapid evolution of multicellular life.

Earth's crust was also very active during the Proterozoic. There was an increase in crustal recycling through subduction. Subduction is when one plate of the crust slides under another. This process helped the cores of the first continents grow larger. These stable cores are called cratons. About 43% of modern continental crust formed during the Proterozoic. This was much more than the 18% that formed in the Phanerozoic.

Continents moved and crashed together in cycles. This movement created mountain building activity known as orogeny. Geologists see evidence of these cycles in the rock record. One major supercontinent was Rodinia, which existed from 1000 to 750 Ma. Rodinia was attached to a central core called Laurentia. This core is the basis for the North American continent. Later, the collision of several landmasses formed the supercontinent Gondwana.

Climate changed drastically during this eon as well. The Proterozoic saw several global glaciations. One was the Huronian glaciation, which lasted 300 million years. At the end of the eon, scientists hypothesize a "Snowball Earth." This might have happened during the Sturtian and Marinoan glaciations. These extreme cold periods happened during the Cryogenian period. These events show how much the Earth's systems can shift.

Life became much more complex as the eon progressed. Simple cells evolved into eukaryotes through a process called symbiogenesis. This is when different organisms live together in a way that benefits both. This led to the evolution of mitochondria and chloroplasts. By the Ediacaran period, soft-bodied multicellular organisms were abundant. These included sponges, algae, and cnidarians. These fossils provide the first obvious evidence of complex life on Earth.